Transmission control system in electric vehicle

ABSTRACT

A transmission control system in an electric vehicle in which a battery is used as en energy source for a motor and is charged with an electric power generated through the motor by regenerative braking of driving wheels which are connected to the motor through a transmission. In the transmission control system, a regenerative energy capable of being generated at a current transmission ratio, a regenerative energy capable of being generated when such transmission ratio is increased, and a regenerative energy capable of being generated when such transmission ratio is decreased, are calculated and compared with one another, thereby shift-changing the transmission into a shift position which permits the largest regenerative energy to be provided. This ensures that the efficiency of recovery of the energy can be increased to the maximum to effectively charge the battery.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmission control system in anelectric vehicle in which a battery is used as an energy source for amotor and is charged with electric power generated by the motor byregenerative braking of a driving wheel connected to the motor through atransmission.

2. Description of the Prior Art

There are conventionally known transmission control systems in vehiclesdriven to travel by an electric motor using a battery as an energysource, in which the battery is charged with an electric power generatedby the motor through regenerative braking of driving wheels, therebyprolonging the possible travel distance of the vehicle obtained by onecharging (for example, see Japanese Patent Publication No. 6204/81 andU.S. Pat. No.3,621,929).

The magnitude of a regenerative energy provided by a regeneratingbraking varies depending upon a transmission ratio of a transmissionmounted between the driving wheels and the motor. In the vehiclesdescribed in the above prior art references, however, the transmissionratio cannot arbitrarily be changed and hence, the kinetic energy of thevehicle cannot be recovered as regenerative energy without any waste, toeffectively charge the battery.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to increase theefficiency in recovering the energy as much as possible, by regenerativebraking to prolong the possible travel distance obtained by a singlebattery charge, by controlling the transmission during the regenerativebraking.

To achieve the above object, according to the present invention, thereis provided a transmission control system in an electric vehicle inwhich a battery is used as an energy source for a motor and is chargedwith electric power generated by the motor through regenerative brakingof a driving wheel which is connected to the motor through atransmission, the transmission control system comprising transmissionratio determination means for calculating a regenerative energy capableof being generated at a current transmission ratio, a regenerativeenergy capable of being generated when the transmission ratio isincreased, and a regenerative energy capable of being generated when thetransmission ratio is decreased, and comparing results of thesecalculations to determine a transmission ratio at which a largestregenerative energy can be obtained, and transmission control means forchanging the transmission ratio of the transmission into a valuepermitting the largest regenerative energy to be provided, based on atransmission ratio command signal output from the transmission ratiodetermination means.

With the above arrangement, the transmission is operated, such that thetransmission ratio permitting the largest regenerative energy to beprovided is determined and obtained, by calculating the regenerativeenergy capable of being generated at a current transmission ratio andthe regenerative energies capable of being generated when suchtransmission ratio is increased and decreased, and comparing theseregenerative energies with one another. Therefore, it is possible toincrease the efficiency of recovering the energy to the maximum toeffectively charge the battery.

The present invention also has a feature that the transmission ratiodetermination means calculates a regenerative energy corresponding tothe transmission ratio by a product of a motor efficiency, a motortorque and a motor revolution number.

With the above arrangement, a regenerative energy corresponding to eachtransmission ratio is calculated by a product of a motor efficiency, amotor torque and a motor revolution number, and therefore, it ispossible to accurately determine a regenerative energy to perform aproper change in the transmission ratio.

The present invention has a further feature that the transmissioncontrol system further includes brake control means for discontinuingthe regenerative braking and hydraulically braking the driving wheels,when the connection between the driving wheels and the motor is releasedin order to effect shifting based on the transmission ratio commandsignal.

With the above arrangement, the regenerative braking is discontinued andthe driving wheel is hydraulically braked when the connection betweenthe driving wheels and the motor is released for effecting shifting.Therefore, it is possible to prevent the magnitude of the braking forceduring the shifting, thereby enabling smooth braking.

Further, the present invention has a feature that the transmission ratiodetermination means discontinues outputting of the transmission ratiocommand signal during steering of the vehicle.

With the above arrangement, the outputting of the transmission ratiocommand signal is discontinued when the braking is conducted duringsteering. The behavior of the vehicle is therefore prevented from beingunstabilized by the shifting during the steering.

The above and other objects, features and advantages of the inventionwill become apparent from the following description of preferredembodiments, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the entire arrangement of an electric vehicleequipped with a braking system according to an embodiment;

FIG. 2 is a block diagram of a control system;

FIG. 3 is a schematic diagram for illustrating braking modes;

FIG. 4 is a flow chart of a main routine;

FIG. 5 is a flow chart of a subroutine corresponding to a step S300 inthe main routine;

FIG. 6 is a flow chart of a subroutine corresponding to a step S301shown in FIG. 5;

FIGS. 7A, 7B and 7C are graphs attendant on the flow chart shown in FIG.6;

FIG. 8 is a graph attendant on the flow chart shown in FIG. 5;

FIG. 9 is a flow chart of a subroutine corresponding to a step S500 inthe main routine;

FIG. 10 is a flow chart of subroutine corresponding to a step S500 inthe main routine;

FIG. 11 is a flow chart corresponding to a subroutine of a step S509shown in FIG. 9;

FIG. 12 is a flow chart corresponding to a subroutine of a step S515shown in FIG. 10;

FIG. 13 is a graph attendant on the flow chart shown in FIG. 12;

FIG. 14 is a flow chart corresponding to a subroutine of a step S517shown in FIG. 10;

FIG. 15 is a flow chart corresponding to a subroutine of a step S519shown in FIG. 10;

FIG. 16 is a flow chart corresponding to a subroutine of a step S518shown in FIG. 10;

FIGS. 17A, 17B, 17C, and 17D are graphs attendant on the flow chartsshown in FIGS. 14 and 15;

FIGS. 18-20 are flow charts corresponding to a subroutine of a step S600in the main routine;

FIG. 21 is a graph attendant on the flow charts shown in FIGS. 18 to 20;

FIG. 22 is a time chart in the event when a shift change is conductedduring braking;

FIG. 23 is a flow chart corresponding to a subroutine of a step S700 inthe main routine;

FIG. 24 is a flow chart corresponding to a subroutine of a step S900 inthe main routine;

FIG. 25 is a flow chart corresponding to a subroutine of a step S902shown in FIG. 24;

FIG. 26 is a flow chart corresponding to a subroutine of a step S903shown in FIG. 24;

FIG. 27 is a graph attendant on the flow chart shown in FIG. 25;

FIG. 28 is a graph attendant on the flow chart shown in FIG. 26;

FIG. 29 is a flow chart of another embodiment corresponding to FIGS. 9and 10;

FIG. 30 is a flow chart corresponding to a subroutine of a step S582shown in FIG. 29; and

FIG. 31 is a flow chart of a further embodiment corresponding to FIG.25.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described by way of preferredembodiments in connection with the accompanying drawings.

Referring to FIG. 1, an electric vehicle of one embodiment is shown as afour-wheel vehicle having a pair of front wheels Wf as follower wheelsand a pair of rear wheels Wr as driving wheels. The rear wheels Wr areconnected through a forward four-stage transmission 3 and a differential4 to an electric motor 2 with a battery 1 used as an energy source. APDU (power drive unit) 5 is interposed between the battery 1 and themotor 2 to control the driving of the motor 2 by the battery 1 and tocontrol the charging of the battery 1 with electric power generated bythe motor 2 as a result of regenerative braking operations. The PDU 5and the transmission 3 are connected to a motor/transmission control ECU(electronic control unit) 6 which is connected to a brake ECU(electronic control unit) 7.

A master cylinder 9 is operable by a brake pedal 8 and is connected torespective brake cylinders 13f for the front wheels Wf and to respectivebrake cylinders 13r for the rear wheels Wr through a modulator 12connected to an accumulator 11 which accumulates pressure generated by ahydraulic pump 10. The modulator 12 includes two-channel ABS (antilockbrake system) control valves 14f for the front wheels and a one-channelABS control valve 14r for the rear wheels, so that when a lockingtendency is produced in the front and rear wheels Wf and Wr, thehydraulic braking pressure transmitted to the brake cylinders 13f and13r is reduced.

Provided in respective oil passages connecting the master cylinder 9 andthe modulator 12 are a hydraulic pressure control valve arrangementcomprised of a differential pressure control valve 16f and an ON/OFFvalve 15f for controlling the hydraulic braking pressure transmitted tothe brake cylinders 13f for the front wheels Wf, and a hydraulicpressure control valve arrangement comprised of a differential pressurecontrol valve 16r and an ON/OFF valve 15r for controlling the hydraulicbraking pressure transmitted to the brake cylinders 13r for the frontwheels Wr.

The ON/OFF valve 15f for the front wheels is a normally-opened typeon/off valve driven by a solenoid and adapted to cut off thecommunication between the master cylinder 9 and the modulator 12, whenrequired. The differential pressure control valve 16f for the frontwheels is mounted in a bypass oil passage bypassing the ON/OFF valve 15fand comprises a valve member 18f biased in an opening direction by aspring 17f, and a linear solenoid 19f for adjusting the set load of thespring 17f. The ON/OFF valve 15r and the differential pressure controlvalve 16r for the rear wheels have the same structures as those for thefront wheels, respectively. Moreover, a one-way valve is provided in thebypass oil passage for restraining feeding of the hydraulic pressurefrom the master cylinder 9 to the modulator 12, but permitting feedingof the hydraulic pressure from the modulator 12 to the master cylinder9.

As can be seen from FIG. 1 together with FIG. 2, connected to the brakeECU 7 are a battery voltage sensor 20₁, a battery current sensor 20₂, abattery remaining-capacity meter 20₃, and a battery temperature sensor20₄ which are mounted on the battery 1; a motor revolution number sensor22 for detecting the number of revolutions of the motor 2; wheel speedsensors 23 mounted on the front and rear wheels Wf and Wr, respectively;a brake pedal depression force sensor 24₁, and a brake pedal switch 24₂mounted on the brake pedal 8; an accelerator opening degree sensor 25mounted on the accelerator pedal 28; a steering sensor 26 mounted on asteering wheel 29; and an accumulator pressure sensor 27 mounted on theaccumulator 12. Further, connected to the brake EPU 7 are the hydraulicpump 10, the hydraulic pressure control valve arrangements eachcomprised of the ON/OFF valve 15f, 15r and the differential pressurecontrol valve 16f, 16r, and the ABS control valves 14f and 14r, all ofwhich are controlled based on output signals from the above describedsensors.

The PDU 5 for controlling the battery 1 and the motor 2 and thetransmission 3 are connected to the motor/transmission control ECU 6which is operated with a regenerative braking command and a transmissionshift command output from the brake ECU 7.

The outline of various braking modes will be described with reference toFIG. 3.

The braking modes for the front and rear wheels Wf and Wr in the vehicleequipped with the braking system according to this embodiment includethree types: "mode 1", "mode 2" and "mode 3". Any one of these modes isselected by an initial decision, so that the braking is carried out inthe selected mode, and in response to a change in the operationalcondition of the vehicle, the changing of the mode is carried out.

(1) Mode 3

This mode is selected in a usual operational condition. Morespecifically, this mode is selected when a regenerative braking systemis normally operative and when a hard or intense braking is notconducted and the steering wheel is not steered. The mode 3 is a mode inwhich the front wheels Wf are braked by the hydraulic pressure, and therear wheels Wr are braked by the hydraulic pressure and by theregeneration. In this mode, when the brake pedal 8 is depressed,firstly, only the rear wheels Wr are braked in a regenerative manner,while the hydraulic braking of the front wheels Wf is not performed.When the braking force for the rear wheels Wr reaches a fold point P,the hydraulic braking of the front wheels Wf is started from thisinstant. When the braking force for the rear wheels Wr exceeds aregenerative limit determined from various conditions for the battery 1and the motor 2, the rear wheels Wf are braked by combination of theregeneration with the hydraulic pressure. When the braking force for therear wheels Wr reaches a fold point Q, the braking force is reduced byan action of a well-known proportional reduction valve mounted withinthe modulator 12. Eventually, a braking force distributioncharacteristic shown by a fold line OPQR is provided. This braking forcedistribution characteristic OPQR is offset upwardly of a theoretical orideal distribution characteristic shown by a dashed line. In otherwords, the braking force distribution for the rear wheels Wr liesupwardly of the theoretical distribution characteristic. This ensuresthat the battery 1 can be charged by utilizing as much the regenerativebraking of the rear wheels Wr as possible, thereby providingprolongation in a possible travel distance obtained by one charge. Inthe mode 3, however, only for an extremely short time, immediately afterstarting of the braking operation, the front wheels Wf are braked by thehydraulic pressure and the rear wheels Wr are braked by combination ofthe regeneration with the hydraulic pressure. If it is confirmed thatthere is no abnormality in the braking device for such period of time,the braking by the hydraulic pressure is then immediately stopped, andthe rear wheels Wr are braked only by the regeneration.

(2) Mode 2

This mode is selected when the regenerative braking system normallyfunctions with a hard braking being not conducted but with the steeringwheel being steered. Like the above-described mode 3, the mode 2 is amode in which the front wheels Wf are braked by the hydraulic pressureand the rear wheels Wr are braked by the hydraulic pressure and theregeneration. However, if the brake pedal 8 is depressed down, thehydraulic braking of the front wheels Wf is performed simultaneouslywith and in parallel to the regenerative braking of the rear wheels Wr.If the braking force for the rear wheels Wr exceeds the regenerativelimit during such braking, the rear wheels Wr are then braked bycombination of the hydraulic pressure with the regeneration. When thebraking force reaches a fold line R, the braking force for the rearwheels Wr is reduced by means of the proportional reduction valve. Afold line OQR representing the resulting braking force distributioncharacteristic places more weight on the braking force for the frontwheels Wf than that placed by the theoretical distributioncharacteristic shown by the dashed line. In this way, it is possible toavoid a reduction in the handling stability by selecting the mode 2during steering and braking the front and rear wheels Wf and Wrsimultaneously from the initial stage of braking.

(3) Mode 1

This mode is selected when the regenerative braking system does notnormally function, or during a hard braking when the regenerativebraking system normally functions. In this mode 1, the regenerativebraking of the rear wheels Wr is not performed, and both the front andrear wheels Wf and Wr are braked by the hydraulic pressure. Byperforming only the hydraulic braking without the regenerative brakingof the rear wheels Wr in this manner, it is possible to enhance theresponsiveness in application of the braking force, as compared with theregenerative braking wherein a slight delay in responsiveness may beproduced while the rotation of the rear wheels Wr is being transmittedthrough the differential 4 and the transmission 3 to the motor 2. Thebraking force distribution characteristic shown by the fold line OQRplaces more weight on the braking force for the front wheels Wf thanthat placed by the theoretical distribution characteristic shown by thedashed line as in the above-described mode 2. The responsiveness of thebraking is improved by selecting the mode 1 during hard braking, asdescribed above.

When hard braking is effected during the braking operation according tothe mode 3, the mode change from the mode 3 to the mode 1 is achieved.On the other hand, when a steering operation is conducted during thebraking according to the mode 3, or when a wheel locking tendency due toa low friction coefficient (low μ) is detected, the mode change from themode 3 to the mode 2 is performed. When a further marked wheel-lockingtendency due to a low μ road is detected during the braking according tothe mode 2, the mode change from the mode 2 to the mode 1 is performed.In this way, by selecting the mode 2 or the mode 1 depending upon thefriction coefficient (μ) of the road surface, it is possible to avoid areduction in handling stability. The mode change from the mode 3 to themode 2 or to the mode 1 is carried out in alignment with an equalbraking force line, i e., along a line showing that a sum of the brakingforce for the front wheels Wf and the braking force for the rear wheelsWr is kept constant, thereby avoiding a sudden variation in totalbraking force for the front and rear wheels Wf and Wr.

The operation of the braking system having the above-describedconstruction will be described with reference to a flow chart of a mainroutine shown in FIG. 4.

First, at a step S100, various programs and data are stored in memoriesin the brake ECU 7 and the motor/transmission control ECU 6, and thebraking system is initially set in an operable state. At a subsequentstep S200, output signals from the battery voltage sensor 20₁, thebattery current sensor 20₂, the battery remaining-capacity meter 20₃,the battery temperature sensor 20₄, the motor revolution-number sensor22, the wheel speed sensors 23, the brake pedal depression force sensor24₁, the brake pedal switch 24₂, the accelerator opening degree sensor25, the steering sensor 26 and the accumulator pressure sensor 27 areread into the brake ECU 7 (see FIG. 2).

At a step S300, a limit value of the regenerative braking force capableof being exhibited at each instance is calculated based on the outputsignals from the above described various sensors. The limit of theregenerative braking force depends upon the state of the battery 1and/or the state of the motor 2, and the details thereof will bedescribed hereinafter with reference to a subroutine of the step S300.

At a step S400, a regenerative braking force corresponding to anengine-braking is calculated. In a vehicle using an internal combustionengine as a travel power source, if the depressing force on theaccelerator pedal is released, an engine-braking is applied. In avehicle using a motor 2 as a travel power source as in this embodiment,a braking force corresponding to the engine-braking is applied to therear wheels Wr by a regenerative braking, thereby providing a steeringfeeling similar to that in a usual vehicle having an internal combustionengine. More specifically, if the depressing force on the acceleratorpedal 28 is weakened, a braking force corresponding to theengine-braking is calculated based on the accelerator opening degreedetected by the accelerator opening degree sensor 25, the motorrevolution number detected by the motor revolution number sensor 22 andthe wheel speeds detected by the wheel speed sensors 23. In order toprovide such braking force, the wheels Wr connected to the motor 2 areregeneratively braked. An electric power generated by the motor 2 as aresult of the regenerative braking is supplied for the charging of thebattery.

At a step S500, a distribution ratio of the regenerative braking forceto the hydraulic braking force is calculated. In other words, the mode3, 2 or 1 is selected, and depending upon conditions of the braking andsteering operations conducted by a driver, or the friction coefficient,a change in mode from the mode 3 to the mode 2 or the mode 1 may bedetermined. In each of the modes, the magnitude of the hydraulic brakingforce for the front wheels Wf and the magnitudes of the regenerativebraking force and the hydraulic braking force for the rear wheels Wr arecalculated. The detailed content of the step S500 will be describedhereinafter with reference to a subroutine of the step S500.

At a step S600, a shift position capable of exhibiting the regenerativebraking force to the maximum is calculated, and the transmission 3 isautomatically operated toward such shift position. The detailed contentof the step S600 will be described hereinafter with reference to asubroutine of the step S600.

At a step S700, the ON/OFF valves 15f and 15r and the differentialpressure control valves 16f and 16r shown in FIG. 1 are actuallycontrolled in order to distribute the regenerative braking force and thehydraulic braking force at a predetermined ratio. An electric powergenerated by the motor 2 as a result of the regenerative braking of therear wheels Wr is supplied for the charging of the battery 1. Thedetailed content of the step S700 will be described hereinafter withreference to a subroutine of the step S700.

At a step S800, an antilock control is performed to prevent an excessiveslip of the front wheels Wf or the rear wheels wr. More specifically, ifit is detected by the output signal from the wheel speed sensor 23 thata wheel begins to come into a locked state, the ABS control valves 14fand 14r shown in FIG. 1 are controlled. This causes the modulator 12interposed between the master cylinder 9 and the brake cylinders 13f and13r to be operated, thereby reducing the hydraulic braking pressuretransmitted to the braking cylinder 13f, 13r for the wheel which is inthe locking tendency, thus preventing the locking of the wheel.

At a step S900, failure of the braking system is detected. If it isdecided that any failure has occurred, then the normally-opened typeON/OFF valves 15f and 15r are maintained at their opened position,thereby permitting the master cylinder 9 and the modulator 12 to be putinto direct communication with each other. As a result, the mode 1 isselected unconditionally, so that the front and rear wheels Wf and Wrare braked by the hydraulic pressure as in a usual hydraulic brakingsystem.

The detailed content of the step S300 (the calculation of the limitvalue of the regenerative braking force) in the flow chart shown in FIG.4 will be described below with reference to FIGS. 5 to 8.

As shown in a routine for calculation of the limit value of theregenerative braking force in FIG. 5, a limit value T_(LMB) of theregenerative braking force based on the state of the battery 1 is firstcalculated at a step S301, and then, a limit value T_(LMN) of theregenerative braking force based on the number of revolutions of themotor 2 is calculated at a step S302. The magnitudes of the limit valueT_(LMB) and the limit value T_(LMN) are compared at a step S303. If thelimit value T_(LMB) is larger than the limit value T_(LMNH), the smallerlimit value T_(LMN) is selected as a regenerative braking force limitvalue T_(LM) at a step S304. If the limit value T_(LMB) is equal to orsmaller than the limit value T_(LHN) the smaller limit value T_(LMB) isselected as the regenerative braking force limit value T_(LM) at a stepS305. In other words, the regenerative braking force limit value T_(LM)is determined by smaller one of the limit value T.sub. LMB based on thestate of the battery 1 and the limit value T_(LMS) based on the numberof revolutions of the motor 2.

A subroutine of the step S301 (the calculation of the limit value basedon the state of the battery) in FIG. 5 will be described below withreference to FIG. 6.

If it is detected, at a step S311, by the output signal from the brakepedal depression-force sensor 24₁, that the braking operation has beenperformed, a regenerative ON timer RBT starts counting at a step S312.Then, a depth of discharge (DOD) of the battery 1 is calculated based onthe output signal from the battery remaining-capacity meter 20₃ at astep S313.

At subsequent steps S314 to S318, a limit value T_(LMBO) is determinedbased on the magnitude of the DOD. More specifically, if the value ofDOD is small, and the remaining capacity of the battery 1 is large, thelimit value T_(LMBO) is set at a small level. If the value DOD is largeand the remaining capacity of the battery 1 is small, the limit valueT_(LMBO) is set at a large level. This will be described in detail withreference to FIG. 6 together with FIG. 7A. If the DOD is equal to orless than a threshold value D₁ and the remaining capacity of the battery1 is relatively large, then the limit value T_(LMBO) is set at arelatively small level T_(LMBI). If the DOD is equal to or less than athreshold value D₂ and the remaining capacity of the battery 1 isrelatively small, then the limit value T_(LMBO) is set at a relativelylarge level T_(LMB3). the DOD is between the threshold values D₁ and D₂,the limit value T.sub. LMBO is set at a limit value T_(LMB2) which isbetween the value T_(LMBI) and the value T_(LMB3).

At a next step S319, a coefficient K₁ for correcting the limit valueT_(LMBO) based on the output signal from the battery temperature sensor21 is determined. More specifically, the capacity of the battery 1increases with higher temperature and hence, the temperature coefficientK₁ is set equal to a value which increases linearly from 1 as the outputsignal TEMP from the battery temperature sensor 21 exceeds TEMP_(O), asshown in FIG. 7B.

At subsequent steps S320 to S324, a coefficient K₂ for correcting thelimit value T_(LMBO) based on the magnitude of the regenerative ON timet counted by the regenerative ON timer RBT is determined. As is apparentfrom FIG. 7C, if the regenerative ON time t which is a time lapsed fromthe start of the regenerative braking is equal to or less than athreshold value t₁, the charging time coefficient K₂ is set at 1. If theregenerative ON time t exceeds the threshold value t₁, the charging timecoefficient K₂ is set at K₂₂ which is smaller than 1. If theregenerative ON time t is equal to or more than a threshold value t₂,the charging time coefficient K₂ is set at K₂₁ which is even smallerthan K₂₂. In this way, the charging time coefficient K₂ becomes themaximum value of 1 at an initial stage of the charging of the battery 1performed efficiently, and with the lapse of the regenerative ON time t,the charging time coefficient K₂ decreases from 1 to K₂₁ and K₂₂.

At a step S325, a final regenerative braking force limit value T_(LMB)based on the battery 1 is calculated by multiplying the limit valueT_(LMBO) based on the DOD by the temperature coefficient K₁ and thecharging time coefficient K₂.

The calculation of the regenerative braking force limit value T_(LMB) iscarried out at every time when the brake pedal 8 is depressed. When thedepressing force on the brake pedal 8 is released, the regenerative ONtimer is reset at a step S326.

FIG. 8 illustrates the variation in limit value T_(LMN) of theregenerative braking force based on the output signal N_(M) from themotor revolution-number sensor 22, which corresponds to the step S302 inthe flow chart shown in FIG. 5. As apparent from FIG. 8, the limit valueT_(LMN) increases linearly with the increase in number N_(M) ofrevolutions of the motor 1, becomes substantially constant soon, and isthen decreased rapidly.

The detailed content of the step S500 (the calculation of theregenerative and hydraulic distribution) in the flow chart shown in FIG.4 will be described below with reference to FIGS. 9 to 17.

As shown in the regenerative and hydraulic distribution determinationroutine in FIGS. 9 and 10, if the brake pedal switch 24₂ is turned ON bythe start of the braking operation at a step S501, an initial timer INTstarts a count-down at a step S502. Then, until a predetermined time islapsed, an initial flag INFL is set at "1" at a step S503, and after thelapse of the predetermined time, the initial flag INFL is set at "0(zero)" (at steps S503, S504 and S505). When the brake pedal switch 24₂is turned OFF at the step S501, the initial timer INT is reset at a stepS506.

Now, the mode 3 is selected at a step S517 in a non-steering conditionwherein; a mode-1 flag is "0" and the mode 1 is not selected at a stepS507; the regenerative braking system is not out of order at a stepS508; a hard braking is not applied at steps S509 and S510; the frictioncoefficient μ of a road surface is sufficiently large at steps S511 andS512 to causes no wheel locking tendency and as a result, thetime-differentiation value ΔV_(W) of the wheel speed (falling of thewheel speed per unit time) of calculated from the output signal from thewheel speed sensor 23 is equal to or less than predetermined thresholdvalues g₁ and g₂ (g₁ >g₂) ; both of a temporary mode-2 flag M2FL' and amode-2 flag M2FL are "0' and the mode 2 is not selected at steps S513and S514; and a steering flag STRFL is not set at "1" at steps S515 andS516. If the mode-1 flag is set at "1" at the step S507, the mode-1 isselected at the step S518. If the mode-2 flag M2FL is set at "1" at thestep S514, the mode-2 is selected at the step S519.

The mode-1 flag MLFL for selecting the mode 1 is set at "1" at a stepS520, when any one of the following conditions (1) to (4) isestablished:

(1) The case where the regenerative braking system is out of order atthe step S508;

(2) The case where it is decided at the steps S509 and S510 that a hardbraking has been applied;

(3) The case where the time-differentiation value ΔV_(w) of the wheelspeed exceeds the larger threshold value g₁ at the step S511 (g₁ isselected as a value which is evaluated when a wheel is about to becomelocked, even in a braking force distribution of a usual hydraulicbraking system); and

(4) The case where the time-differentiation value {V_(w) of the wheelspeed is between the larger threshold value g₁ and the smaller thresholdvalue g₂ (g₂ is selected as a value which is evaluated when the lockingtendency is eliminated, if the braking force distribution characteristicis returned to the braking force distribution of the usual hydraulicbraking system), and where the mode-2 flag M2FL is set at "1" at thestep S521.

The temporary mode-2 flag M2FL' for deciding the selection of the mode 2is set at "1" at a step S527, when the following condition (5) isestablished, and likewise, the mode-2 flag M2FL is set at "1" at a stepS522, when the following condition (6) or (7) is established:

(5) The case where the time-differentiation value ΔV_(w) of the wheelspeed exceeds the smaller threshold value g₂ at the step S512, and themode-2 flag M2FL is not set at "1" at the step S521 (i.e., a state inwhich the mode 3 has been selected), and moreover, a M2 direct timer M2Tis counting down at steps S523 and S524;

(6) The case where the time-differentiation value ΔV_(w) of the wheelspeed becomes equal to or less than the smaller threshold value g₂ atthe step S512 during counting-down of the M2 direct timer M2T at stepsS523 and S524 (i.e., M2FL'=1) , or a predetermined time counted by theM2 direct timer M2T is lapsed. It should be noted that even after thelapse of the predetermined time counted by the M2 direct timer M2T, ifit is decided at the step S512 that ΔV_(w) >g₂, the mode 1 is selectedbased on the above-described condition (4); and

(7) The case where it is decided at the steps S515 and S516 that thesteering is being conducted.

Both of the mode-2 flag M2FL and the mode-1 flag MLFL are not set at "0"at steps S528 and S529, until it is decided at the step S510 that thebraking operation is not conducted, i.e., until the depressing force onthe brake pedal 8 is released. Therefore, once the mode 2 or the mode 1is selected during one braking, the mode cannot be shifted back from themode 2 or 1 to the mode 3 during such braking.

The detail of the step S509 (judgement of the hard braking) in FIG. 9will be described below with reference to a flow chart shown in FIG. 11.If the depression force F_(B) detected by the brake pedal depressionforce sensor 24₁ is equal to or more than a predetermined thresholdvalue at a step S531, it is decided unconditionally that the hardbraking is being applied, and a hard-braking flag is set at "1" at astep S532.

On the other hand, if the depression force F_(B) is less than thepredetermined threshold value and a hard-braking judging timer PTM is atan initial value t₀ at the start of counting, the current depressionforce F_(B) is brought into an initial depression force F_(B1) at a stepS534. If the hard-braking judging timer PTM does not count down to 0(zero) at a subsequent step S535, the count-down is performed at a stepS538, and the hard-braking flag is set at "0" at a step S537.

When the hard-braking judging timer PTM completes the countdown to 0(zero) at the step S535, i.e., when the predetermined time t₀ is lapsed,the current depression force F_(B) is brought into a t₀ -laterdepression force t_(B2) at a step S538, and the hard-braking judgingtimer PTM is reset at t₀ at a step S539. At a next step S540, adifference between the t₀ -later depression force t_(B2) and the initialdepression force F_(B1) is compared with a threshold value ΔF_(B) ofvariation in depression force. If the difference exceeds the depressionforce variation threshold value ΔF_(B), the hard-braking flag is set at∓1". If the difference does not exceed the depression force variationthreshold value ΔF_(B), the hard-braking flag is set at "0".

In this way, if the depression force F_(B) exceeds a first thresholdvalue, and if an increment in depression force F_(B) within apredetermined time exceeds a second threshold value, then it is decidedthat the hard braking is being applied.

The detail of the step S515 (judgement of steering conditions) in FIG.10 will be described below with reference to a flow chart in FIG. 12 anda graph in FIG. 13. If the vehicle speed V calculated from the outputsignals of the wheel speed sensors 23 is larger than a largest thresholdvalue V₃, then the steering flag STRFL is set at "1" when the steeringangle θ detected by the steering sensor 26 is larger than a smallestthreshold value θ₁, and the steering flag STRFL is set at "0" when thesteering angle θ is equal to or smaller than the smallest thresholdvalue θ₁ (see steps S541, S542, S543, S544, S545 and S546) .

If the vehicle speed V is between the largest threshold value V₃ and athreshold value V₂ smaller than the largest threshold value V₃, then thesteering flag STRFL is set at "1" when the steering angle θ is largerthan a mean threshold value θ₂, and the steering flag STRFL is set at"O" when the steering angle θ is equal to or smaller than the thresholdvalue θ₂ (see the steps S541, S542, S543, S544, S545 and S546).

If the vehicle speed V is between the threshold value V₂ and a smallestthreshold value V₁, then the steering flag STRFL is set at "1" when thesteering angle θ is larger than the largest threshold value θ₃, and thesteering flag STRFL is set at "0", when the steering angle θ is equal toor smaller than the threshold value θ₃ (see the steps S541, S542, S543,S544, S545 and S546).

If the vehicle speed V is equal to or smaller than the smallestthreshold value V₁, then the steering flag STRFL is set at "0",irrespective of the magnitude of the steering angle θ (see the stepsS541 and S546).

In this way, when the velocity of the vehicle is high, it is decidedthat the steering is being conducted, even if the steering angle θ issmall. When the velocity of the vehicle is low, it is not decided thatthe steering is being conducted, unless the steering angle θ is large.

The detail of the step S517 (determination of the mode-3 distribution)in FIG. 10 will be described below with reference to a flow chart inFIG. 14 and a graph in FIGS. 17A, 17B, and 17C. At a step S551, areduced regenerative braking force limit value T_(RGLM) converted into atire torque is calculated by multiplying the regenerative braking forcelimit value T_(LM) determined at the step S300 in FIG. 4 by a gear ratioR(n) in an n-th gear shift. At a next step S552, a depression forceF_(BO) corresponding to the fold point P (at which the hydraulic brakingof the front wheels Wf is started) in the braking force distributioncharacteristic in FIG. 1 is determined based on a graph in FIG. 17A.

At a step S553, a reduced regenerative braking force T_(RG)corresponding to the depression force F_(B) is determined based on agraph shown in FIG. 17B. At a next step S554, an Fr offset quantity,i.e., a quantity of operation of the linear solenoid 19f is calculatedby multiplying the depression force F_(BO) at the fold point by aconstant. At a step S555, an Rr offset quantity, i.e., a quantity ofoperation of the linear solenoid 19r shown in FIG. 1 is determined basedon a graph shown in FIG. 17C. If the initial flag INFL (see the stepsS504 and S505) is set at "1" at a subsequent step S556, i.e., if apredetermined time is not lapsed from the start of braking, then both ofan Fr offset flag and an Rr offset flag for controlling the ON/OFFvalves 15f and 15r shown in FIG. 1 are set at "0" (the valves areopened) at a step S557. On the other hand, if the initial flag INFL isset at "0 " at the step S556, i.e., if the predetermined time has beenlapsed from the start of braking, then both of the Fr offset flag andthe Rr offset flag are set at "1" (the valves are closed) at the stepS557.

The details of the step S519 (the determination of a mode-2distribution) shown in FIG. 10 will be described below with reference toa flow chart shown in FIG. 15 and the graph shown in FIG. 17D. At a stepS561, a reduced regenerative braking force limit value T_(RGLM)converted into a tire torque is calculated by multiplying theregenerative braking force limit value T_(LM) determined at the stepS300 in FIG. 4 by a gear ratio R(n) in an n-th gear shift. At a nextstep S562, a reduced regenerative braking force T_(RG) is determinedbased on a graph shown in FIG. 17D.

At a step S563, the Fr offset quantity is set at "0". This is becausethe braking force distribution characteristic in the mode 2 does nothave the fold point P, and the hydraulic braking of the front wheel Wfis conducted from the initial stage of braking. At a next step S564, anRr offset quantity, i.e., a quantity of operation of the linear solenoid19r shown in FIG. 1 is searched based on the graph shown in FIG. 17C. Ata step S565, the Fr offset flag for controlling the ON/OFF valve 15f forthe front wheels is set at "0" (the valve is opened) , and the Fr of fset f lag f or controlling the ON/OFF valve 15r for the rear wheels isset at "1" (the valve is closed).

The details of the step S518 (the determination of a mode-1distribution) shown in FIG. 10 will be described below with reference toa flow chart shown in FIG. 16. In this mode 1, the regenerative brakingis not performed and for this reason, the reduced regenerative brakingforce T_(RG) is set at "0" at a step S571. At subsequent steps S572 andS573, both of the Fr offset quantity and the Rr offset quantity are setat "0". Finally, at a step S574, both of the Fr offset flag and the Rroffset flag are set at "0" (the valves are opened), and a hydraulicpressure generated by the master cylinder 9 is transmitted in an intactmanner to the modulator 12, thereby causing the front and rear wheels Wfand Wr to be braked by a usual hydraulic pressure.

The details of a step S600 (the commanding of shifting) shown in FIG. 10will be described below with reference to flow charts shown in FIGS. 18to 20, a graph shown in FIG. 21 and a time chart shown in FIG. 22.

If the shifting is being conducted at a step S601 in the flow chartsshown in FIGS. 18 to 20, the shift flag SHFL is set at "1" at a stepS602. Then, the reduced regenerative braking force T_(RG) is set at "0"at a step S603, and both of the Fr offset flag and the Rr offset flagare set at "0" (the valves are opened) at a step S604. This causes thefront and rear wheels Wf and Wr to be braked by a usual hydraulicpressure without the regenerative braking during the shifting.

If the shift flag SHFL is set at "1" at a step S605, notwithstandingthat the shifting is not conducted at the step S601, it is decided thatthe shifting has been completed. At a subsequent step S606, thehydraulic braking of the front and rear wheels Wf and Wr is released,and at a step S607, the shift flag SHFL is set at "0".

If the steering is being conducted and the steering flag STRFL is set at"1" at a step S608, the commanding of shifting which will be describedhereinafter is not performed.

At subsequent steps S609 to 613, an estimated regenerative electricpower E(n) in the current shift position n is calculated. Morespecifically, at the step S609, a motor torque T_(MT)(n) in an n-th gearshift is calculated by dividing the reduced regenerative braking forceT_(RG) by a gear ratio R (n) . Then, at the step 612, a motor efficiencyη (n) is determined from the motor torque T_(MT)(n) and the number N_(M)of revolutions of the motor 2 based on a graph shown in FIG. 21, and atthe next step S613, the estimated regenerative electric power in thecurrent shift position n is calculated by multiplying the motorefficiency η (n) by the motor torque T_(MT)(n) and the number N_(M) ofrevolutions of the motor 2.

Then, an estimated regenerative electric power E.sub.(n-1) in the eventthat a downshifting from the current shift position is performed iscalculated at steps S614 to 622. More specifically, if the current shiftposition n is a first gear shift at the step S614, the downshifting issubstantially impossible and hence, the estimated regenerative electricpower E.sub.(n-1) at the downshifting is set at "0" at the step S615. Ifthe current shift position n is any of second to fourth gear shifts atthe step S614, an estimated regenerative electric power E.sub.(n-1) inthe event when the downshifting to an n-1-st gear shift is conducted islikewise calculated at the steps S616 to S622. In this case, if themotor torque T_(MT)(n-1) exceeds the regenerative braking force limitvalue T_(LM) at the step S617, the regenerative braking force limitvalue T_(LM) is equalized to the motor torque T_(MT)(n-1) at the stepS618. In the case of the downshifting, the number N_(M)(n-1) ofrevolutions of the motor 1 at the downshifting performed is calculatedfrom the gear ratios R_(n) and R.sub.(n-1) and the revolution numberN_(m)(n) in the n-th gear shift. If the revolution number N_(M)(n-1) isconsequently a number corresponding to over-revolutions at the stepS620, the estimated regenerative electric power E.sub.(n-1) is set at"0" at the step S615.

Subsequently, an estimated regenerative electric power E.sub.(n+1) inthe event when an upshifting is conducted from the current shiftposition is calculated at steps S623 to S630. More specifically, if thecurrent shift position n is a fourth gear shift at the step S623, theupshifting is substantially impossible and hence, the estimatedregenerative electric power E.sub.(n+1) in the event when an upshiftingis conducted is set at "0" at the step S624. At the subsequent stepsS625 to S630, an estimated regenerative electric power E.sub.(n+1) inthe event when an upshifting is conducted is likewise calculated. Inthis case, if the motor torque T_(MT)(n+1) exceeds the regenerativebraking force limit value T_(LM) at the step S626, the regenerativebraking force limit value T_(LM) is set equal to the motor torqueT_(MT)(n+1) at the step S627. It should be noted that in the case of theupshifting, over-revolutions cannot be produced and hence, the judgementof the over-revolutions as carried out in the case of the downshiftingis not carried out.

The current estimated regenerative electric power E.sub.(n), theestimated regenerative electric power E.sub.(n-1) at the downshiftingperformed and the estimated regenerative electric power E.sub.(n+1) atthe upshifting performed are compared with one another at steps S631 to633. If the E.sub.(n-1) is largest, a downshifting command is issued ata step S634. Reversely, if the E.sub.(n+1) is largest, an upshiftingcommand is outputted at a step S635.

The above-described shifting operation will be described with referenceto time chart shown in FIG. 22. Suppose that the operation is conducted,for example, so that the depression force on the brake pedal 8 isgradually increased at times T₁, T₃ and T₈, and a regenerative brakingcommand is outputted at a time T₂. In this case, if it is decided thatthe shift position is downshifted, for example, from the third gearshift to the second gear shift to maximize the regenerated energy, theclutch is disengaged at a time T₄.

When the clutch is disengaged, the rear wheels Wr and the motor 2 aredisconnected f rom each other, and the regenerative braking becomesimpossible. Therefore, the regenerative braking command to the motor 2is canceled from the time T₄ to a time T₇. For a period in which theregenerative braking is not conducted, i.e., for a period from the timeT₄ to the time T₇, a hydraulic braking command is output, so that theregenerative braking is replaced by the hydraulic braking. At a time T₅in a period of disengagement of the clutch from the time T₄ to a timeT₆, a shifting command is output, so that the downshifting from thethird gear shift to the second gear shift is carried out.

In the above manner, the total braking force is ensured by theregenerative braking for a period from the time T₂ to the time T₄, bythe hydraulic braking for a period from the time T₄ to the time T₇, bythe regenerative braking for a period from the time T₇ to a time T₉ andby a combination of the regenerative braking and the hydraulic brakingafter the time T₉.

The details of the step S700 (the control of regenerative and hydraulicbraking forces) shown in FIG. 4 will be described with reference to aflow chart shown in FIG. 23.

First, a reduced regenerative braking force T_(RG) is delivered at astep S701. Then, as can be seen from FIG. 1, the rear wheels Wr areregeneratively braked in the mode 3 and the mode 2, so that the reducedregenerative braking force T_(RG) is obtained during initial braking,and electric power generated by the motor 2 as a result of suchregenerative braking is supplied for charging. The Fr offset quantityand the Rr offset quantity determined at the steps S554 and S555 shownin FIG. 14 and corresponding to the mode 3, at the steps S563 and S564shown in FIG. 15 and corresponding to the mode 2 and at the steps S572and S573 shown in FIG. 16 and corresponding to the mode 1 are deliveredat steps S702 and S703. As a result, the linear solenoids 19f and 19r ofthe hydraulic cylinders for the front and rear wheels Wf and Wr shown inFIG. 1 are operated to adjust the preset loads of the springs 17f and17r of the differential pressure control valves 16f and 16r to apredetermined magnitude.

If the Fr offset flag is not set at "1" at a subsequent step S704 (inthe case of the mode 2 and the mode 1), the ON/OFF valve 15f shown inFIG. 1 is maintained opened at a step S705. Conversely, if the Fr offsetflag is set at "1" (in the case of the mode 3), the ON/OFF valve 15f isclosed at a step S706.

Thus, in the modes 2 and 3 in which the ON/OFF valve 15f is maintainedopened, a hydraulic braking pressure generated by the master cylinder 9is transmitted directly to the modulator 12, so that the front wheels Wfare hydraulically braked from the time of an initial braking, as shownin the braking force distribution characteristics in the mode 2 and themode 1 shown in FIG. 3.

On the other hand, in the mode 3, the ON/OFF valve 15f is closed andhence, the hydraulic braking pressure generated by the master cylinder 9is blocked by the ON/OFF valve 15f and transmitted via the differentialpressure control valve 16f to the modulator 12. In this case, thedifferential pressure control valve 16f is not opened, until thedepression force on the brake pedal 8 is increased to a predeterminedvalue by the preset load of the spring 17f. As a result, the applicationof the hydraulic braking force to the front wheels Wf at the initialbraking is inhibited, as shown by the line OP in the braking forcedistribution characteristics in the mode 3 shown in FIG. 3. When thehydraulic braking pressure generated by the master cylinder 9 becomes amagnitude corresponding to the fold point P, the differential pressurecontrol valve 16f is opened, so that the hydraulic braking force isstarted to be applied to the front wheels Wf.

Returning to the flow chart shown in FIG. 23, if the Rr offset flag isnot set at "1" at a step S707 (in the case of the mode 1), the ON/OFFvalve 15r is maintained opened at a step S708. Reversely, if the Rroffset flag is set at "1" (in the case of the mode 3 and the mode 2),the ON/OFF valve 15r is closed at a step S709.

Thus, in the mode 1 in which the ON/OFF valve 15r is maintained opened,the hydraulic braking pressure generated by the master cylinder 9 istransmitted directly to the modulator 12, so that the rear wheels Wr arehydraulically braked from the time of the initial braking, as shown inthe braking force distribution characteristic in the mode 1 shown inFIG. 3.

On the other hand, in the mode 3 and the mode 2, the ON/OFF valve 15r isclosed and hence, the hydraulic braking pressure generated by the mastercylinder 9 is blocked by the ON/OFF valve 15r and transmitted via thedifferential pressure control valve 16r to the modulator 12. In thiscase, the differential pressure control valve 16r is not opened, untilthe depression force on the brake pedal 8 is increased to apredetermined value by the preset load of the spring 17r. As a result,the application of the hydraulic braking force to the rear wheels Wr toregeneration limits in the braking force distribution characteristics inthe mode 3 and 2 shown in FIG. 3 is inhibited. When the hydraulicbraking pressure generated by the master cylinder 9 becomes a magnitudecorresponding to such regeneration limit, the differential pressurecontrol valve 16r is opened, so that the hydraulic braking force isstarted to be applied to the rear wheels Wr and thereafter, theregenerative braking force and the hydraulic braking force are appliedto the rear wheels Wr.

Now, at the start of braking, the mode 3 is first selected for usualbraking, as described in the flow charts shown in FIGS. 9 and 10. Inthis case, if a failure of the regenerative braking system, hardbraking, or a special condition such as steering, is detected, the modeis shifted from mode 3 to mode 1 or mode 2. However, if only theregenerative braking of the rear wheels Wr is conducted immediatelyafter the start of braking in the mode 3, there is a possibility of adelayed response due to a rise-time of the hydraulic pressure. In orderto prevent this, the initial flag INFL is set at "1" for an extremelyshort period of time until the initial timer INT adapted to start thecountdown simultaneously with the start of braking reaches the time-up,as given in the steps S502 to S505 shown in FIG. 9. As a result, both ofthe Fr offset flag and the Rr offset flag are set at "0", as given inthe steps S558 and S557. Therefore, in addition to the regenerativebraking of the rear wheels Wr in the usual mode 3, the hydraulic brakingof the front wheels Wf and the hydraulic braking of the rear wheels Wrby opening of the ON/OFF valves 15f and 15r are simultaneously carriedout for a predetermined period of time immediately after the start ofbraking.

When the mode is shifted from the mode 3 immediately after the start ofbraking to the mode 1 or the mode 2, the hydraulic braking can bestarted without any delay to avoid the delay of response of the braking.It is of course that when the mode 3 is selected again, the temporaryhydraulic braking is discontinued after the time-up of the initial timerINT and turned to the regenerative braking of the rear wheels Wr in themode 3.

When the brake pedal 8 is depressed from an accelerating condition toeffect a braking, a slight delay of response is produced until theregenerative braking begins to become actually effective, due to abacklash or a distortion present in a power transmitting system fortransmitting a driving force from the motor 2 through the transmission3, the differential 4 and a drive shaft to the rear wheels Wr. However,the delay of response of the regenerative braking can be minimized byproducing the hydraulic braking simultaneously with the depression ofthe brake pedal 8, as described above.

The details of a step S900 (judgement of fails) shown in FIG. 4 will bedescribed below with reference to flow charts shown in FIGS. 24 to 26and graphs shown in FIGS. 27 and 28.

If any trouble is not produced and the fail flag FAILFL (see the stepS508 shown in FIG. 9) is not set at "1" at a step S901 in the flow chartshown in FIG. 24, the failure of the regenerative braking system, thebrake pedal depression force sensor 24₁, the steering sensor 26, thewheel speed sensors 23, the ABS control valves 14f and 14r, thehydraulic pressure control valves, i.e., the ON/OFF valves 15f and 15rand the differential pressure control valves 16f and 16r, and thehydraulic pump 10 are detected sequentially at subsequent steps S902 to908.

A subroutine of the step S902 (judgement of regenerative fails) shown inFIG. 24 will be described below with reference to a flow chart shown inFIG. 25.

First, at a step S911, an actual regenerative electric power E_(RG)generated by the motor 2 is calculated by multiplying an output signalV_(B) from the battery voltage sensor 20₁ and an output signal I_(B)from the battery current sensor 20₂ by each other. At subsequent stepsS912 and S913, it is determined whether or not the actual regenerativeelectric power E_(RG) is between two values resulting from the additionand subtraction of a predetermined value α to and from the estimatedregenerative electric power E.sub.(n) calculated at the step S613 shownin FIG. 18, i.e., between E.sub.(n) +α and E.sub.(n) -α. If the actualregenerative electric power E_(RG) and the estimated regenerativeelectric power E.sub.(n) are in the obliquely lined regions shown inFIG. 27, it is decided that there is an abnormality in the regenerativebraking system.

If the judgement of abnormality has been first conducted and a temporaryfail flag FAILFL' is not set at "1" at a step S914, the temporary failflag FAILFL' is set at "1" at a step S915. When it is decided again in anext loop that there is an abnormality in the regenerative brakingsystem, i.e., when the temporary fail flag FAILFL' has been set at "1"at the step S914, the fail flag FAILFL is finally set at "1" at a stepS916. When it is decided at the steps S912 and S913 that theregenerative braking system is normal, the temporary fail flag FAILFL'and the fail flag FAILFL are set at "0" at steps S917 and S918. If thetemporary fail flag FAILFL' is set at "1" at the step S915, the failflag FAILFL is set at "0" at the step S918.

If the actual regenerative electric power E_(RG) is excessively large,so as to exceed a predetermined value, or excessively small bycomparison of the actual regenerative electric power E_(RG) with theestimated regenerative electric power E.sub.(n) and it is decided thatthere is an abnormality, the temporary fail flag FAILFL' is set. If theabnormality is also continuously detected in a next loop, the fail flagFAILFL is set, thereby enabling the failure of the regenerative brakingsystem to be reliably detected without any influence such as electricalwaves which may invade from the outside of the control system, such asfrom the outside of the vehicle.

A subroutine 21 of the step S903 (the judgement of a fail of the brakepedal depression force sensor) shown in FIG. 24 will be described belowwith reference to the flow chart shown in FIG. 26.

First, at steps S921 and S922, it is judged whether of not the outputsignal from the brake pedal depression force sensor 24₁ is between 0.4 Vto 4.6 V. As shown in the graph shown in FIG. 28, the output V_(FB) fromthe brake pedal depression force sensor 24₁ is set so that it isincreased linearly from 0.5 V to 4.5 V, as the depression force F_(B) isincreased, and thereafter, such output is maintained constant at 4.5 V.An acceptable range of error of the brake pedal depression force sensor24₁ is from -0.1 V to 0.1 V and hence, if the brake pedal depressionforce sensor 24₁ is normal, the output V_(FB) should be between theminimum value of 0.4 V and the maximum value of 4.6 V. Therefore, if theoutput V_(FB) is not between 0.4 V and 4.6 V at the steps S921 and S922,it is decided that there is an abnormality in the brake pedal depressionforce sensor 24₁.

If the judgement of abnormality has been first performed and thetemporary fail flag FAILFL' is not set at "1" at a step S923, thetemporary fail flag FAILFL' is set at "1" at a step S924. If it isdecided again in a next loop that there is an abnormality in the brakepedal depression force sensor 24₁, i.e., if the temporary fail flagFAILFL' is set at "1" at the step S924, then the fail flag FAILFL isfinally set at "1" at a step S925.

If the output V_(FB) from the brake pedal depression force sensor 24,exceeds 0.6 Vat a step S927, notwithstanding that the brake pedal switch24₂ is not turned ON at a step S926, i.e., notwithstanding that thebrake pedal 8 is not operated, it is decided that there is anabnormality in the brake pedal depression force sensor 24₁, passing intothe step S923.

If it is decided at the steps S921, S922, S926 and S927 that the brakepedal depression force sensor 24₁ is normal, both of the temporary failflag FAILFL' and the fail flag FAILFL are set at "0" at steps S928 andS929. If the brake pedal switch 24₂ is turned ON at the step S926 and ifthe temporary fail flag FAILFL' is set at "1" at the step S924, the failflag FAILFL is set at "0" at the step S929.

As described above, it is decided that there is an abnormality, if theoutput V_(FB) from the brake pedal depression force sensor 24₁ indicatesan impossible value (equal to or less than 0.4 V and equal to or notmore than 4.6 V) when the brake pedal 8 is operated, and if the outputV_(FB) from the brake pedal depression force sensor 24₁ indicates animpossible value (equal to or more than 0.6 V). Therefore, it ispossible to reliably detect not only an abnormality of the output fromthe brake pedal depression force sensor 24₁ but also a failure due to asticking of the brake pedal depression force sensor 24₁. Moreover, it isfinally decided that there is an abnormality, when the abnormality iscontinuously detected by use of the temporary fail flag FAILFL'.Therefore, it is possible to reliably detect the failure of the brakepedal depression force sensor 24₁ without any influence such aselectrical waves which may invade from the outside of the controlsystem, such as from the outside of the vehicle.

FIG. 29 illustrates another embodiment, in which the regenerative andhydraulic pressure distribution determining routine shown in FIGS. 9 and10 is simplified.

In this embodiment, if the regenerative braking system is not out oforder, if the steering is not conducted, if the depression force is notequal to or more than a threshold value and if the friction coefficientof a road surface is not low, the usual mode 3 is selected (see thesteps S581, S582, S583, S584, S585, S586 and S587).

If the regenerative braking system is out of order at the step S581, themode 1 is selected unconditionally at the step S589. If it is decided atthe steps S582 and S583 that the steering is being conducted, if it isdecided at the step S584 that the depression force has become equal toor more than the threshold value and a hard braking is applied, or if itis decided at the steps S585 and S586 that the friction coefficient of aroad surface is low, so that there is a possibility of the locking ofthe wheel, the mode 2 is selected at the step S588.

FIG. 30 illustrates a subroutine of the step S582 (the judgement of asteering condition) shown in FIG. 29. In this subroutine, if the lateralacceleration of the vehicle is equal to or higher than a reference valueat a step S591, it is decided that the steering is being conducted, andthe steering flag STRFL is set at "1" at a step S592. If the lateralacceleration is lower than the reference value, it is decided that thesteering is not conducted, and the steering flag STRFL is set at "0" ata step S593. Alternatively, the steering condition can be judged using ayaw rate as a parameter in place of the lateral acceleration of thevehicle at the step S591.

FIG. 31 illustrates an alternate embodiment of the regenerative failjudging routine shown in FIG. 25.

First, at a step S931, the reduced regenerative braking force T_(RG)(see the step S556 shown in FIG. 14 and the step S562 shown in FIG. 15)converted into a tire torque is compared with a reference reducedregenerative braking force T_(RGO) corresponding to the maximum electricpower consumed by loads of electric equipments. If the reducedregenerative braking force T_(RG) exceeds the reference reducedregenerative braking force T_(RGO) thereby enabling the loads to becompensated for by the regenerated electric power, it is judged at astep S932 whether an electric current value I_(B) delivered by thebattery current sensor 20₂ is plus or minus. If the electric currentvalue I_(B) is minus and the battery 1 is being charged, it is decidedthat there is an abnormality, and the temporary fail flag FAILFL' andthe fail flag FAILFL are operated at steps S933, S934 and S935 in thesame manner as described above.

If normal at the step S931 and S932 (if YES at the step S931 and No atthe step S932), the processing is advanced to a step S936. If theelectric current value I_(B) is plus at the step S937 and the battery isbeing charged, notwithstanding that the reduced regenerative brakingforce T_(RG) is 0 (zero) and the regenerative braking is not conducted,it is also decided that there is an abnormality, passing to the stepS933. It should be noted that the temporary fail flag FAILFL' and thefail flag FAILFL are reset at steps S938 and S939 in the same manner asat the steps S917 and S918 shown in FIG. 25.

Although the embodiments of the present invention have been describedabove, it will be understood that the present invention is not limitedto these embodiment, and various minor modifications in design can bemade without departing from the spirit and scope of the invention.

For example, although the vehicle having the front wheels Wf as followerwheels and the rear wheels Wr as driving wheels is shown and describedby way of example, the present invention is applicable to a vehiclehaving front wheels as driving wheels and rear wheels as followerwheels. In addition, although the vehicle in the embodiments includesthe gear type transmission as a transmitting device, the presentinvention is applicable to a vehicle including a continuously variabletransmission such as CVT in place of the above-described transmission.

What is claimed is:
 1. A transmission control system in an electric vehicle in which a battery is used as an energy source for a motor and is charged with an electric power generated by said motor through regenerative braking of a driving wheel which is connected to said motor through a transmission, said transmission control system comprisingtransmission ratio determination means for calculating a regenerative energy capable of being generated at a current transmission ratio, a regenerative energy capable of being generated when said transmission ratio is increased, and a regenerative energy capable of being generated when said transmission ratio is decreased, and comparing results of these calculations to determine a transmission ratio at which a largest regenerative energy can be obtained, and transmission control means for changing said transmission ratio of said transmission into a value permitting said largest regenerative energy to be provided, based on a transmission ratio command signal output from said transmission ratio determination means.
 2. A transmission control system in an electric vehicle according to claim 1, wherein said transmission ratio determination means calculates a regenerative energy corresponding to said transmission ratio by a product among a motor efficiency, a motor torque and a motor revolution number.
 3. A transmission control system in an electric vehicle according to claim 1, further including brake control means for discontinuing said regerenative braking and hydraulically braking the driving wheel, when the connection between the driving wheel and the motor is released in order to effect a shifting based on said transmission ratio command signal.
 4. A transmission control system in an electric vehicle according to claim 1, wherein said transmission ratio determination means discontinues outputting of said transmission ratio command signal during steering of the vehicle.
 5. Apparatus for braking an electric vehicle, comprisingmeans for braking by means of regeneration force; means for determining a preferred transmission ratio for operating said means for braking; and means for altering a transmission ratio for said vehicle in response to said means for determining.
 6. Apparatus as in claim 5, wherein said means for determining comprisesmeans for calculating a first amount of regeneration force to be provided for a present transmission ratio and a second amount of regeneration force to be provided for at least one other transmission ratio; and means for comparing said first amount of regeneration force and said second amount of regeneration force.
 7. Apparatus as in claim 5, wherein said means for determining comprisesmeans for calculating a first amount of regeneration force to be provided for a first transmission ratio and a second amount of regeneration force to be provided for a second transmission ratio; and means for comparing said first amount of regeneration force and said second amount of regeneration force.
 8. Apparatus as in claim 5, wherein said means for determining is responsive to a motor speed sensor. 